The present invention relates generally to the ability to impart surface textures on composite materials for use in the construction industry. More particularly, the present invention relates to the use of lightweight, flexible dies to impart surface texture on composite materials when the composite materials are still in a semi-slurry state.
The United States Gypsum Company's gypsum fiberboard process, as shown and described in U.S. Pat. No. 5,320,677, and herein incorporated by reference in its entirety, describes a composite product and a process for producing a composite material in which a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the gypsum, i.e. Calcium sulfate in the stable dihydrate state (CaSO.sub.4.multidot.2H.sub.2 O), to calcium sulfate alpha hemihydrate having acicular crystals. The cellulosic fibers have pores or voids on the surface and the alpha hemihydrate crystals form within, on and around the voids and pores of the cellulosic fibers. The heated slurry is then dewatered to form a mat, preferably using equipment similar to paper making equipment, and the slurry cools enough to begin rehydrating the hemihydrate to gypsum, whereupon the mat is pressed into a board of the desired configuration. The pressed mat undergoes an exothermic reaction and rehydrates to gypsum to form a dimensionally stable, strong and useful building board. The board is thereafter trimmed and dried.
One of the many advantages of the process disclosed in the '677 patent is that a surface texture can be imparted on the resulting gypsum panel as the panel is being formed. Two examples of boards of this type are textured panels for manufactured housing applications and surface relief panels for a variety of markets.
The challenge in surface texturing gypsum fiberboard during in-line processing is the timing of the impression made on the slurry or wet mat. As the rehydration begins and solidification of the mass starts to occur, an exothermic reaction takes place. Firstly after calcination there is a cooling of the mat as the slurry is dewatered, such as by vacuum extraction and a primary press arranged along the moving conveyor belt or screen. The dewatering primary press is used as a first press to eliminate up to approximately 90% of the free water remaining after vacuum extraction. Before rehydration, it is important to eliminate usually about 80-90% of free water while bringing the temperature of the filter cake down. Dewatering processes contribute significantly to lowering the filter cake temperature. Extracting free water is necessary when seeking to texture and wet press the filter cake into a desired product shape. Alternatively, the filter cake could be immediately dried and then cooled to a stable but rehydratable hemihydrate for later use. It is therefore also desirable to remove as much of the free water that is not required in the composite mass for rehydration before the temperature drops to the rehydration temperature.
Upon reaching the rehydration temperature, which may require additional cooling, an exothermic reaction takes place. The exothermic reaction results in a hydration curve which is plotted as temperature over time, or distance along the conveyor. As the rehydratable calcium sulfate hemihydrate and cellulosic fibers in a slurry form leave the head box, the hemihydrate crystals will have a temperature generally in the range of about 180.degree. F. to about 210.degree. F. Thereafter, the slurry is spread across the conveyor and the action of vacuum pumps begins removal of the free water and the temperature drops significantly. The rehydration temperature on the conveyor can vary depending on the additives and accelerators used, but is generally in the range from about 60.degree. F. to about 120.degree. F. This would plot as the low or starting point on the hydration, or so-called temperature trace, curve. At this point, the exothermic reaction ensues and heat is released. The temperature plot will show an increasing curve until a substantially constant slope (linear) plot of increasing temperature over time, or distance, is reached. The exothermic reaction will then taper resulting in a graphic change from an upward linear slope to a curved plot that reaches a peak temperature, signaling a decrease in hydration rate. Thereafter, the curve slopes downwardly as the reaction winds down to reach 100% hydration. Ultimately, the board may be dried to eliminate any excess water.
The critical key for imparting texture is finding where on the temperature curve between the inception of hydration to its termination should texturing occur so that a) the texturing does not end too soon for the setting composite to hold the relief, b) the forming acicular gypsum crystal structures are not destroyed, and c) the impression is not imparted so late that the surface texture is broken by having been too firmly set to receive texture.
The usual method of choice for imparting textures on wide panels is by using a roll to texture a moldable surface, such as is done with wet felted ceiling tile. However, fabrication of such rolls typically has long lead times and high costs. Another option is to make flat sheets and then glue them on to the roll. Unfortunately, for both methods, fabricated rolls then have little opportunity to change the texturing pattern. Roll processes heretofore have not proved highly successful.
A third roll method is the fabrication of rubber sleeves over KEVLAR brand para-aramid or nickel, which may then be slid on or off of a mandrel, generally using compressed air. This method allows texture changes using less expensive sleeves over a common mandrel, yet still has long lead times for initial fabrication.
A non-roll option, which is commonly used for embossed hardboard and some cement board products, involves machining a steel platen, laying it against a surface and applying sufficient pressure and or heat in a platen press to impart the texture to a panel surface. These imparted surface are generally very high quality. The steel platens have the added advantage of making it easy to change textured patterns, as long as a different platen pattern is in stock. However, this method requires difficult and unwieldy equipment that is associated with the handling of steel platens, especially with larger size panels. In addition, such large steel platen dies tend to be expensive.
Deep patterns, such as wood grained panels or wainscot panels, may be made in at least one of four ways. Wood molding may be cut and attached to paneled products. The disadvantage to this method is cost and the time associated with the finishing of molded corners and edges as well as maintaining uniformity of panels. Uniformity may be increased by using a roll to impart the texture to a moldable surface, such as is done with wet felted ceiling tile. However, fabrication of such rolls typically has long lead times and high costs. Deeper features, such as the molding of wainscot panels, require more machining with higher cost and even longer lead times. Such rolls then have little opportunity to change the embossing pattern. A third option involves machining a steel platen, laying it against a surface and applying sufficient pressure and or heat to impart the texture to a panel surface as described above. A fourth method is machining the profile or relief into the surface of the panel, which gives a rougher surface and generates substantial dust that must be collected, handled and disposed of, or recycled.